Most of the chain saws in use, when properly maintained and prepared, do perform the job they have been fitted for in a satisfactory way. Yet they have a shortcoming that cuts the actual working hours. In the dark time of the year, in particular, the effective working hours of a forest worker are limited into the lightest hours of the day, that is six hours a day more or less. The chain saw is used in cutting and delimbing a tree, and the effective working hours consumed in these operations are a decisive factor in the whole process. If the chain or the cutter blade as a whole has to be replaced in the forest, as is frequently the case, work efficiency is crucially affected. The chain may be damaged or broken or the chain has to be tightened at times. Besides, it would often be of advantage to use different chain and different blade length in cutting and delimbing. The chain saws presently used fail to allow for such considerations to a sufficient extent. They are composed of a number of separate parts that must be used and handled whenever the cutter blade is changed or adjusted. Such parts include all the following: chain, blade, side cover, bolts and chain tensioner. The saw has to be disassembled far enough to call it a time-consuming and inconvenient process in field conditions.